Methods for producing silver nanoparticles

ABSTRACT

An exemplary embodiment of the invention is a method for making silver nanoparticles, and includes steps of reacting a silver salt with a phosphene amino acid to make silver nanoparticles. Exemplary phosphene amino acids include trimers, with a particular example being a trimeric amino acid conjugate containing one phosphene group. In an exemplary method of the invention, the silver nanoparticles may be produced in timer periods of less than about 30 minutes, and at temperatures of less than about 40° C. Other methods of the invention are directed to methods for stabilizing silver nanoparticles.

FIELD OF THE INVENTION

A field of the invention is methods for producing silver nanoparticles.

BACKGROUND

Silver nanoparticles have a multitude of valuable applications in therapidly emerging fields of nanoscience and nanotechnology. Powerfulsurface plasmon absorption of nanoparticulate silver makes themparticularly useful in applications such as biosensors, for example.Silver nanoparticles are a photo-fluorescence marker, which makes themuseful for a number of medical and similar applications. They areenvironmentally and biologically benign. Other exemplary silvernanoparticle applications include smart windows, rewritable electronicpaper, electronic panel displays, memory components, and others.

Traditional methods for the production of silver nanoparticles requireuse of potentially harmful chemicals such as hydrazine, sodiumborohydride and dimethyl formamide (“DMF”). These chemicals posehandling, storage, and transportation risks that add substantial costand difficulty to the production of silver nanoparticles. A highlytrained production workforce is required, along with costly productionfacilities outfitted for use with these potentially harmful chemicals.

These harmful chemicals also make it impractical, if not impossible, toproduce silver nanoparticles in-vivo. This limitation results in silvernanoparticles having to be prepared beforehand, sanitized, and thenintroduced to a body for many medical applications. These extra stepsadd cost and effort. Also, the complexity of handling silvernanoparticles for these applications further limits their use in suchapplications.

Another disadvantage of known methods for producing silver nanoparticlesrelates to the time and heat required for their production. Knownmethods of production utilize generally slow kinetics, with the resultthat reactions take a long period of time. The length of time requiredmay be shortened by some amount by applying heat, but this adds energycosts, equipment needs, and otherwise complicates the process. Knownmethods generally require reaction for 20 or more hours at elevatedtemperatures of 60°-80 C., for example. The relatively slow kinetics ofknown reactions also results in an undesirably large particle sizedistribution and relatively low conversion. The multiple stages ofproduction, long reaction times at elevated temperatures, relatively lowconversion, and high particle size distribution of known methods makethem costly and cumbersome, particularly when practiced on a commercialscale.

These and other problems with presently known methods for making silvernanoparticles are exacerbated through the relatively unstable nature ofthe nanoparticles. Using presently known methods, the silvernanoparticles produced have only a short shelf life since they tend toquickly agglomerate.

As a result of these and other problems, unresolved needs remain in theart.

SUMMARY OF THE INVENTION

An exemplary embodiment of a method for making silver nanoparticlesincludes steps of providing a silver slat, providing a phosphene aminoacid, and reacting the silver salt with the phosphene amino acid to makesilver nanoparticles. Exemplary phosphene amino acids include trimers,with a particular example being a trimeric amino acid conjugatecontaining one phosphene group. In exemplary methods of the invention,high conversion is achieved in relatively short times and at relativelylow temperatures.

Another exemplary embodiment of the invention is directed to a methodfor stabilizing silver nanoparticles and includes steps of combining aphosphene amino acid with silver nanoparticles. Preferably the phospheneamino acid is a trimeric amino acid conjugate containing one phosphenegroup.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Exemplary methods of the present invention include methods for makingsilver nanoparticles. Exemplary methods of the invention generallyinclude the steps of reacting a silver salt such as AgNO₃ with aphosphene amino acid. Methods of the invention have been discovered tooffer numerous and valuable advantages over the prior art. For example,the silver salt and phosphene amino acid reactants are environmentallyand biologically benign materials that do not require special handlingor storage. Silver nanoparticles may be produced through methods of theinvention in time periods as short as 5 mins. or less and at roomtemperature. These and other advantages will be apparent to thoseskilled in the art when considering the detailed description ofexemplary methods of the invention that follow.

A method of the invention includes reacting a silver salt with aphosphene amino acid, with one particular exemplary method including thesteps of performing the reaction:

where:

-   -   X=OH, Cl, Br, I or NO₃    -   R′=Hydrogen, alkyl (C1-C₆), or amino protecting group    -   R″=OR^(A), NR^(A)R^(B) or R^(C); where R^(A)=R^(B)=hydrogen,        alkyl, phenyl, benzyl, or a carboxyl protecting group; or        R^(A)=R^(B)=pyrollidino, piperdino, or thiomorpholinno ring; and        R^(C)=alkyl, phenyl or benzyl    -   Y=Residue of an amino acid.        Although a trimer amino acid is illustrated and is preferred, a        dimer, polymer, or a monomer is also contemplated for use.

The phosphene amino acid is preferably a conjugate amino acid. Aparticular phosphene amino acid found to be useful in methods of theinvention is a trimer amino acid conjugate that contains trimericalanine and one phosphine group (“TAAC”):

TAAC is described in WIPO International Application No. PCT/US03/05678,Publication No. WO 03/072053, “Compounds for treatment of copperoverload,” with inventors Katti, Kavita K.; Kannan, Raghuraman; Casteel,Stan W.; Katti, Kattesh V; as well as in “Characterization ofSupramolecular (H₂O)18 Water Morphology and Water-Methanol(H₂O)15(CH₃OH)₃ Clusters in a Novel Phosphorus Functionalized TrimericAmino Acid Host,” by Raghuraman, K.; Katti, K. K.; Barbour, L. J.;Pillarsetty, N.; Barnes, C. L.; Katti, K. V.; J. Am. Chem. Soc.; 2003;125(23); 6955-6961. Preferably at least about 1 mole of trimericphosphine amino acid is provided per five moles of silver salt. Fordimeric phosphine amino acid, preferably at least about one mole ofdimer phosphine amino acid is provided per three moles of silver salt,and preferably at least about one mole of monomer phosphene amino acidper two moles of silver salt.

The reaction for the formation of silver nanoparticle is quantitative.The reaction preferably proceeds with at least about 98% formation ofsilver nanoparticles. There are substantially no byproducts—thephosphene amino acid is oxidized during the reaction to yield acorresponding oxide, which is further consumed for assisting theconversion of the silver salt to Ag nanoparticles. It is believed thatthe reduction of silver salt is initiated by phosphine, and that thephosphine in turn is oxidized to phosphine oxide. After the initialstep, and when using TAAC, the aminocarboxylates in the TAAC oxide serveas the reducing agent to reduce silver salt to silver nanoparticles.

Phosphene amino acids useful in methods of the invention, with TAACbeing one example, are environmentally and biologically benign compoundsthat are stable and easily handled. As such, their use offerssubstantial advantages over methods of the prior art that requirehazardous, biologically/environmentally unfriendly reactants that aremore difficult and costly to store and handle.

To aid the reaction, it is preferred that a solvent such as water isprovided, along with a stabilizer such as starch. The reaction proceedssubstantially to completion in no more than about 30 mins. at roomtemperature. It is believed that the reaction between silver salt andTAAC proceeds to completion in less than about 5 mins. at roomtemperature, and may occur substantially instantaneously. Theseproduction times represent a substantial improvement over methods of theprior art that required elevated temperatures and relatively longreaction times. Depending on stirring, temperature, and otherconditions, however, other periods of time may be useful to carry out amethod of the invention. Time periods of up to about 30 mins. or about 1hour, for example, may be useful to insure maximum conversion. Someelevation in temperature above room temperature may also be useful toinsure maximum completion and to speed reaction times, although hightemperatures are not necessary. By way of example, the invention may bepracticed at temperatures of less than about 40° C., or less than about30° C.

Methods of the invention also offer substantial improvements inconversion of silver salt to silver nanoparticles. Conversion tonanoparticles of at least about 70% of the silver contained in thesilver salt, for example, may be achieved in time periods of less thanabout 1 hour, in less than about 30 mins, or even in less than about 5mins. depending on conditions that include concentrations of reactantspresent, temperature, stirring, and the like. Higher conversions arealso possible in methods of the invention, with at least about 90%conversion or 98% more preferred, and in time periods of less than about30 mins, and more preferably less than about 5 mins. It will beappreciated that the practice of the present invention at relatively lowtemperatures, short reaction times, and high conversion rates offersimportant advantages and benefits over the prior art.

Still another valuable advantage of methods of the invention is that thesize distribution of resultant silver nanoparticle is relatively tightlydefined, and can be at least partially tuned. By way of example, atleast about 80% of the silver nanoparticles produced through a method ofthe invention may have a size range of between about 3-5 nm, while inanother method of the invention at least about 80% may be between about10-20 nm. In general, silver nanoparticles of size 3-30 nm arepotentially useful for many medicinal and industrial applications.Methods of the present invention may be practiced to deliver at leastabout 80%, and more preferably at least about 90% silver nanoparticleshaving this size.

Desired size ranges may be achieved by varying the concentration ofphosphene amino acid present. Taking TAAC as an example, it includesmolecular cavities of about 5 nm in size. To increase the number ofsmall sized silver nanoparticles, the amount of TAAC present isincreased. Silver nanoparticles are then formed primarily in thecavities. To increase the number of large particle size silvernanoparticles, the amount of TAAC may be decreased to result in arelatively high amount of small particles. When less TAAC is present, agreater proportion of silver nanoparticles are formed on the surface ofthe TAAC as opposed to in the 5 nm cavities, resulting in a largeraverage particle size. Empirical testing can be performed to determinerequired amounts of phosphene amino acid present to yield a desired sizerange of silver nanoparticles.

It will be appreciated that methods of the invention thereby provideimportant and valuable benefits over the prior art. For example, becausesilver nanoparticles can be produced at room temperature, in shortperiods of time, at high conversion rates, and without the need forhazardous or environmentally/biologically unfriendly reactants, methodsof the invention are particularly well suited to in-vitro and in-vivopractice. Silver nanoparticles may be made, for example, in a livingorganism such as a mammal. By way of example, it may be desirable tomake use of the marking or tracing properties of silver nanoparticlesfor medicinal, research, or other purposes in a human being or ananimal. In such cases, a silver salt could be dispersed in an area ofinterest (during a surgery on an organ, for instance), with a phospheneamino acid solution then introduced in the area. Silver nanoparticleswould result. Likewise, a patient might ingest one or both of thereactants so that silver nanoparticles would be produced in the mouth,throat, stomach, or digestive tract as desired. Further, it may bepractical to rely on the phosphene amino acids present in proteins toproduce silver nanoparticles by introducing a silver salt.

Other applications in which methods of the invention may find utility ismilitary or commercial applications in which it is desired to producesilver nanoparticles quickly, on-site in the field and through a simpleprocedure. A soldier in combat or a field service technician, forinstance, could potentially tear open a two-compartment foil packet witha small amount of an AgNO₃ solution in one compartment and a smallamount of TAAC in the second. Combining the two materials in an area ofinterest would produce silver nanoparticles there for later tracking ordetection. Also, benefits of the invention including high conversionrates, low reaction temperatures, relatively uniform particledistribution size, and easily handled materials lend themselves well toeconomical large-scale commercial production and storage.

It has also been discovered that the phosphene amino acid solutions usedto make silver nanoparticles through methods of the invention provide asubstantially improved storage medium for storing the nanoparticles.Silver nanoparticles may be stabilized (or passivated) by (i) phosphineoxide (e.g., TAAC oxide), (ii) aminocarboxylates (e.g., amino acids fromTAAC), and (iii) hydroxyl groups present in starch. Stabilization occurswhen weak functional groups from any of these sources bind silvernanoparticles. These weakly bound functional groups can also be easilyexchanged with donor ligands such as thiols and/or amines. Amines and/orthiols bearing proteins (or aminoacids) can be bioconjugated to silvernanoparticles by this method. The nanoparticles may be stored forperiods of weeks or months without appreciable agglomeration. By way ofparticular example, silver nanoparticles made through a method of theinvention were stored for a period of 2 weeks and for a period of 6 mos.in the TAAC solution with minimal to no agglomeration.

In order to further describe the present invention, detailed exemplaryprocedures for making silver nanoparticles are presented.

Exemplary Procedure #1

0.1875 gm of starch was added to 50 ml DI water and heated to about 100°C. to dissolve the starch

In a separate container, 0.0337 gm of TAAC was dissolved in 1 ml DIwater.

A silver salt solution was prepared at room temperature by dissolving0.039 gm of AgNO₃ in 1 ml of DI water.

In a separate container, 100 μl of the silver salt solution was added to6 ml of the starch solution with stirring at room temperature.

20 μl of the TAAC solution was added to AgNO₃/starch solution withstirring at room temperature. The color changes to yellow-brownimmediately.

Stirring was continued for about 30 minutes.

This exemplary process of the invention resulted in the production ofsilver nanoparticles having a size in the range of 10 nm. Thenanoparticles were stored in the reaction medium, which contains TAACoxide, amino carboxylates of TAAC, and hydroxyl groups of starch in DIwater, and found stable for more than 30 days.

Exemplary Procedure #2

A saturated solution of starch was prepared by heating 50 ml of DI watercontaining 0.1875g of Starch.

In separate vials, 1M solutions of TAAC and silver nitrate were preparedin dI water.

In a fresh 20 ml sample vial equipped with a magnetic stirrer, 6 ml ofsaturated solution of starch was added, followed by 100 μl of 1Msolution of AgNO₃.

20 μl of 1M TAAC solution was added to the AgNO₃/Starch solution slowly.The color changes to yellow-brown immediately.

Stirring was continued for 30 minutes.

Other embodiments of the invention are directed to methods forstabilizing silver nanoparticles. One exemplary method includes steps ofcombining a phosphene amino acid with silver nanoparticles to stabilizethe silver nanoparticles. The phosphene amino acid is preferably any ofthose described above, with monomers, dimers, and trimers beingexamples. A preferred example is TAAC. Preferably at least about 1 moleof TAAC is provided per mole of silver nanoparticles. At least about 3moles of dimer phosphene amino acid, and at least about 2 moles ofmonomer amino acid are provided per mole of silver nanoparticles.

While specific embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives will be apparent to one knowledgeable inthe field involved. For example, while methods of the invention havebeen described using a particular sequence of steps, it will beappreciated that unless specifically noted other sequences may bepossible. Also, it will be appreciated that in some circumstances acorresponding salt may be used in place of an acid—it will beappreciated that as used herein the term “acid” encompassescorresponding salts. Such modifications, substitutions and alternativescan be made without departing from the spirit and scope of theinvention, which should be determined from the appended claims. Variousfeatures of the invention are set forth in the appended claims.

1. A method for producing silver nanoparticles comprising the steps of:providing a silver salt; providing a phosphene amino acid; reacting saidsilver salt with said phosphene amino acid to make silver nanoparticles.2. The method of claim 1 wherein said phosphene amino acid is a trimer.3. The method of claim 1 wherein said phosphene amino acid is a dimer.4. The method of claim 1 wherein said phosphene amino acid is a trimericamino acid conjugate.
 5. The method of claim 4 wherein said trimericamino acid contains trimeric alanine and one phopshine group.
 6. Themethod of claim 1 and further including the step of mixing saidphosphene amino acid and said silver salt in at least one solvent. 7.The method of claim 1 and further including the step of combining starchand at least one solvent with said silver salt and said phosphene aminoacid.
 8. The method of claim 1 wherein the step of reacting said silversalt with said phosphene amino acid to make silver nanoparticles isperformed at a temperature of less than about 40° C.
 9. The method ofclaim 1 wherein the step of reacting said silver salt with saidphosphene amino acid to make silver nanoparticles is performed at atemperature of less than about 30° C.
 10. The method of claim 1 whereinthe step of reacting said silver salt with said phosphene amino acid tomake silver nanoparticles is performed at substantially roomtemperature.
 11. The method of claim 1 wherein the step of reacting saidsilver salt with said phosphene amino acid converts at least about 70%of the silver in said silver salt is into said silver nanoparticles andis performed in a time period of less than about 1 hour.
 12. The methodof claim 1 wherein the step of reacting said silver salt with saidphosphene amino acid converts at least about 70% of the silver in saidsilver salt is into said silver nanoparticles and is performed in a timeperiod of less than about 30 mins and at room temperature.
 13. Themethod of claim 12 wherein said time period is less than about 5minutes.
 14. The method of claim 1 wherein the step of reacting saidsilver salt with said phosphene amino acid converts at least about 98%of the silver in said silver salt is into said silver nanoparticles andis performed in a time period of less than about 30 mins.
 15. The methodof claim 1 wherein the step of reacting said silver salt with saidphosphene amino acid creates silver nanoparticles at least about 80% ofwhich have a size between about 10-20 nm.
 16. The method of claim 1wherein the method further includes varying the amount of said phosphineamino acid to determine the size distribution of said silvernanoparticles.
 17. The method of claim 1 wherein said phosphene aminoacid comprises trimeric amino acid conjugate, and wherein the step ofproviding a phosphene amino acid includes providing a greater amount ofsaid trimeric amino acid conjugate if a relatively small silvernanoparticle size is desired, and providing a larger amount of saidtrimeric amino acid conjugate if a relatively large silver nanoparticlesize is desired.
 18. The method of claim 1 wherein said silver saltcomprises silver nitrate.
 19. The method of claim 1 and furtherincluding the step of storing said silver nanoparticles for a period ofat least about 2 weeks without any substantial agglomeration of saidnanoparticles.
 20. The method of claim 19 wherein said period is atleast about 6 mos.
 21. The method of claim 1 wherein the method iscarried out within a living organism.
 22. The method of claim 21 whereinsaid living organism is a mammal.
 23. The method of claim 21 whereinsaid living organism is a human being, and wherein the step of reactinga silver salt with a phosphene amino acid further includes introducingsaid silver salt and said phosphene amino acid to a selected area ofsaid human being during a surgical procedure.
 24. The method of claim 1wherein said phosphene amino acid is a dimer, and wherein the step ofreacting said silver salt with said phosphene amino acid comprisesreacting at least about 1 moles of said phosphene amino acid per 3 molesof said silver salt.
 25. The method of claim 1 wherein said phospheneamino acid is a trimer, and wherein the step of reacting said silversalt with said phosphene amino acid comprises reacting at least about 1mole of said phosphene amino acid per 5 moles of said silver salt. 26.The method of claim 1 wherein the step of reacting said silver salt withsaid phosphene amino acid comprises: preparing a silver salt solution;and, combining a phosphene amino acid solution and a starch with saidsilver salt solution with mixing at a temperature of less than about 30°C. to convert at least about 80% of said silver in said silver salt tosilver nanoparticles within about 30 mins.
 27. The method of claim 1wherein said phosphine amino acid comprises:

where: R′=Hydrogen, alkyl (C1-C₆), or amino protecting group R″=OR^(A),NR^(A)R^(B) or R^(C); R^(A)=R^(B)=hydrogen, alkyl, phenyl, benzyl, or acarboxyl protecting group; or R^(A)=R^(B)=pyrollidino, piperdino, orthiomorpholinno ring; R^(C)=alkyl, phenyl or benzyl; and Y=Residue of anamino acid.
 28. A method for producing silver nanoparticles comprisingthe steps of: dissolving starch in water to form a starch solution;dissolving a trimeric amino acid conjugate containing one phosphenegroup in water to form a phosphine amino acid solution; dissolving asilver salt in water to form a silver solution; combining at least aportion of said silver salt solution with at least a portion of saidstarch solution with stirring to form a silver salt starch solution;and, combining at least a portion of said phosphene amino acid solutionwith said silver salt starch solution at a temperature of less thanabout 40° C. with stirring to form silver nanoparticles in less thanabout 30 mins.
 29. A method for storing silver nanoparticles comprisingthe steps of: providing silver nanoparticles; providing a phospheneamino acid; and, combining said silver nanoparticles with said phospheneamino acid.
 30. A method as defined by claim 29 and further includingthe step of storing said silver naoparticles for a period of at leastabout 2 weeks without substantial agglomeration.
 31. A method as definedby claim 29 wherein said phosphene amino acid is a trimeric amino acidconjugate containing one phosphene group.
 32. A method as defined byclaim 29 wherein said phosphene amino acid comprises a trimer and atleast about 1 mole of said phopshene amino acid are provided per mole ofsilver nanoparticles.